8 “Before the Lord, for He has come; for He has come to judge the earth”. “The hills” signify the great. The Lord comes to judge the earth, and they rejoice. But there are hills, who, when the Lord is coming to judge the earth, shall tremble. There are therefore good and evil hills; the good hills, are spiritual greatness; the bad hills, are the swelling of pride. “Let the hills be joyful together before the Lord, for He has come; for He has come to judge the earth.”
Wherefore shall He come, and how shall He come? “With righteousness shall He judge the world, and the people with equity”. Let the hills therefore rejoice; for He shall not judge unrighteously. When some man is coming as a judge, to whom the conscience cannot lie open, even innocent men may tremble, if from him they expect a reward for virtue, or fear the penalty of condemnation; when He shall come who cannot be deceived, let the hills rejoice, let them rejoice fearlessly; they shall be enlightened by Him, not condemned; let them rejoice, because the Lord will come to judge the world with equity; and if the righteous hills rejoice, let the unrighteous tremble.
But behold, He has not yet come: what need is there they should tremble? Let them mend their ways, and rejoice. It is in your power in what way you will to await the coming of Christ. For this reason He delays to come, that when He comes He may not condemn you. Lo, He has not yet come: He is in heaven, thou on earth: He delays His coming, do not thou delay wisdom. His coming is hard to the hard of heart, soft to the pious. See therefore even now what you are: if hard of heart, you can soften; if you are soft, even now rejoice that He will come. For you are a Christian. Yea, you say, I believe that you pray, and sayest, “Your kingdom come.” You desire Him to come, whose coming you fear. Reform yourself, that you may not pray against yourself.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)